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Colicin E2 Is a DNA Endonuclease

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Colicin E2 Is a DNA Endonuclease Proc. Nati. Acad. Sci. USA Vol. 73, No. 11, pp. 3989-S993'November 1976 Biochemistry Colicin E2 is a DNA endonuclease (colicin-E2-immunity protein/colicin E3/colicin-E3-immunity protein) KLAUS SCHALLER AND MASAYASU NOMURA University of Wisconsin, Institute for Enzyme Research, 1710 University Avenue, Madison, Wisc. 53706 Communicated by Henry Lardy, September 10, 1976 ABSTRACT Colicin E2 purified by conventional methods immunity protein (20). These authors demonstrated that the contains a tightly bound low-molecular-weight protein, as has immunity protein could be separated from the E3 protein by been found with purified colicin E3 [Jakes, N. & Zinder, N. D. preparative electrophoresis in sodium dodecyl sulfate/poly- (1974) Proc. Nat. Acad. Sci. USA 71, 3380-33841. Such E2 preparations do not cause DNA cleavage in vitro. After sepa- acrylamide gels and that E3 protein (called "ES*") free of ration from the low-molecular-weight protein, colicin E2 re- immunity protein was much more active than the "complexed" tained the original in vivo killing activity, and in addition ES preparation in ribosome inactivation in vitro (20). They also showed a high activity in vitro in cleaving various DNA mole- noted the presence of a small-molecular-weight protein in cules, such as a ColE1 hybrid plasmid and DNAs from Esche- highly purified E2 preparations (cited in ref. 20). From the richia coli, X phage, 4X174 phage, and simian virus 40. The low-molecular-weight protein ("E2-immunity protein") specif- analogy of the colicin E3-immunity protein complex, they ically prevented this in vitro DNA cleavage reaction, i.e., had inferred that this small-molecular-weight protein is probably an "immunity function." The results demonstrate that colicin the E2-immunity protein. We have now succeeded in sepa- E2 itself is a DNA endonuclease and explain the in vivo effects rating colicin E2 (called "E2*") from the low-molecular-weight caused by E2 in sensitive cells as well as the mechanism of im- protein and demonstrated that E2* thus obtained is highly munity in E2-colicinogenic cells. active in degrading DNA in vitro and that the small-molecu- Colicin E2 (E2) is a protein antibiotic synthesized by certain lar-weight protein prevents this DNase activity, i.e., has an strains of coliform bacteria that carry the ColE2 plasmid, and "immunity function." is classified in the E group of colicins along with colicins El and E3 (1, 2). Although the physical properties of E2 and E3 are MATERIALS AND METHODS similar (3) and both colicins share the same receptor (1, 2), their E. coli PR13, which lacks RNase I, was used as the colicin- apparent mode of action is different. It was first discovered that sensitive strain. Colicins E3 and E2 were purified from mi- E2 causes degradation of DNA, while colicin E3 (E3) causes tomycin-C-induced cultures of E. coli CA38 and W3110 specific inhibition of protein synthesis in sensitive Escherichia (ColE2), respectively, according to the method described by coli cells treated with these colicins (4). Subsequent work on the Herschman and Helinski (3). The colicins obtained were still mode of action of E3 has demonstrated that ES inactivates ri- complexed with immunity proteins (cf. ref. 20). E3-immunity bosomes in treated cells (5) by causing the cleavage of a frag- protein was previously purified by J. Sidikaro in this laboratory ment from the 3' end of the 16S RNA (6, 7). The same cleavage (21). Pure E2-immunity protein was obtained as described reaction has also been demonstrated in vitro using "purified" below. E3 protein (see below) and purified ribosomes (8, 9). This in Colicin E2* was separated from the immunity protein by vitro demonstration of E3 action has strongly indicated that E3 dissociation of the complexed E2 with guanidine-HCl at about acts directly on ribosomes rather than indirectly and that E3 4°. Lyophilized E2 was dissolved in dissociation buffer (1 itself is an RNase with a very stringent substrate specificity. mg/ml). The dissociation buffer contained 6 M guanidine-HCl In contrast to ES, the mechanism of action of E2 has not been (Grade I, Sigma Chemical Co.), 0.2 M NaCl, 1 mM di- clear. Since the initial discovery of DNA degradation in E2- thiothreitol, and sufficient 1 M K2HPO4 to adjust the pH to 6.8. treated sensitive cells (4), several authors have confirmed and After the incubation for 1 hr at 40, the solution was subjected extended the original observations on DNA cleavage reactions to one of two methods to obtain a complete separation of E2* (10-13). However, others have suggested that the primary ac- from the immunity protein. In the first method, the solution tion of E2 is not on the cellular DNA, but on some other targets was applied to a Sephadex G-100 "superfine" column equili- such as membranes (14) or tRNA (15). brated with the dissociation buffer and the two proteins were Since E2 has properties similar to E3 (3), and E3 causes separated. In the second method, the solution was filtered ribosome inactivation in vitro, one might expect some kind of through a PM30 membrane (Amicon) using a 50 ml stirred cell. effects of E2 on cellular DNA in vitro, if DNA is in fact the The protein left in the Amicon cell (E2*) was diluted with the primary target of E2. Several attempts have been made to dissociation buffer and the filtration process was repeated. After demonstrate such effects in vitro using purified E2 (e.g., refs. five such processes, the E2* preparation was diluted in a solu- 16-19). However, published results were either negative or too tion of 6 M urea and 0.2 M NaCl, concentrated again to about uncertain to allow a definitive conclusion regarding such direct 2 mg/ml of protein, and dialyzed against 0.1 M sodium phos- effects. phate, pH 7.0, containing 0.2 M NaCI. The filtrate containing Recently, Jakes and Zinder observed that the "purified" ES the E2-immunity protein was concentrated using a UM2 filter preparations obtained with the conventional purification (Amicon) and treated in the same way as E2*. The proteins methods (3) contained about one molar equivalent of the E3- could be stored in the dialysis buffer at 40 in polypropylene tubes for at least 2 months without loss of in vivo or in vitro Abbreviations: E2 and ES, colicins E2 and ES, respectively; Col, a activity. Colicin ES* was prepared in a similar way. colicinogenic plasmid; kb, kilobase (1000 base pairs); SV40, simian virus Colicin activity in vivo was determined by spot-testing 40. dilutions of colicin in 10 mM potassium phosphate containing 3989 Downloaded by guest on October 2, 2021 3990 Biochemistry: Schaller and Nomura Proc. Natl. Acad. Sci. USA 73 (1976) Table 1. In vivo killing activity of colicins described in Materials and Methods. Jakes and Zinder were unable to recover active E3* after 6 M guanidine.HCl treat- E3 E3* E2 E2* ment and hence used sodium dodecyl sulfate to obtain E3* (20). ± 1 ± 0.5 1 ± 0.5 Using E3* prepared by our method (compare Fig. lb), we have 1 0.5 1 0.5 confirmed the observation made by Jakes and Zinder (20) that Values given are activity units per ng of colicin as defined by E3* is much more active in vitro (about 40-fold) than the Helinski (3). original E3 preparation. In addition, we have observed that the killing activity of E3* was similar to that of the original E3 2 mg/ml of bovine serum albumin on an agar layer of sensitive preparation when tested in vivo by the spot-test method (Table cells (3). To follow the in vitro DNA cleavage reaction, E2* was 1). In contrast to our results, Jakes.and Zinder noted that their incubated with DNA (usually covalently closed circular pLC E3* is 50-500 times less active in vivo than the original E3 (20). 21-9 plasmid DNA) at 370, and samples were analyzed using We conclude that the difference between our results and their slab gel electrophoresis with 0.9% agarose (22). The incubation results is probably due to the difference in the method of mixture contained 20mM Tris-HCI, pH 8.0,80mM NaCl, and preparation of E3* and that the E3-immunity protein is not 10 mM MgSO4. required for killing of intact cells by E3* (see Discussion). pLC 21-9 plasmid DNA was prepared by M. Kenerley in this laboratory from a derivative of E. coli strain MV12 (F+, recA -, Degradation of DNA by E2* thr-, leu-, thi-, Atrp E5) which carries this plasmid. The plasmid was originally constructed by Clarke and Carbon by We have applied the guanidine.HCl dissociation method to connecting an E. coli DNA fragment to ColEl DNA (23). The prepare E2* free of the presumed E2-immunity protein. Fig. method used to prepare this plasmid DNA was similar to that 1 shows the results of polyacrylamide/urea gel analysis of the used to make ColE3 DNA (21). Electron microscopic studies original E2 preparation (gel d), E2* (gel e), and the presumed showed that the plasmid DNA is 12,400 base pairs (12.4 kb) in immunity protein (gel f). We have found that E2* thus pre- length and consists of ColEl DNA (6.5 kb) and bacterial DNA pared degrades various DNAs in vitro as analyzed by agarose (M. Kenerley and M. Nomura, unpublished experiments). The gel electrophoresis. In addition, we also demonstrated that the DNA was mostly in the covalently closed circular form (see, e.g., presumed E2-immunity protein has in fact the ability to inhibit Figs. 2 and 4).
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